The present invention relates to a SQUID (Superconducting Quantum Interference Device) integrating pickup coils which can highly sensitively detect a magnetic field generated from a subject under testing, and more particularly, to a magnetic field measurement system using the SQUID integrating pickup coils. More specifically, the present invention relates to a biomagnetometer which uses a multichannel SQUID integrating pickup coils for measuring a biomagentic field generated by neural activities of a human brain, myocardium activities of a human heart, and so on, and a magnetic field measurement system which uses a multichannel SQUID integrating pickup coils for conducting a non-destructive evaluation.
Generally, a SQUID gradiometer for measuring a biomagnetic field is comprised of axial pickup coils formed of superconducting wires, and a SQUID formed of a thin film which is superconductively connected to the pickup coils. To reduce environmental magnetic field noise, a gradiometer is often used as a pickup coil for detecting a field gradient in a direction of a detected magnetic field. When the detected magnetic field is oriented in the normal direction (z-direction), the gradiometer detects a z-gradient of a z-component (xcex94Bz/xcex94z) of the detected magnetic field (Prior Art 1: J. Clarke, Proceedings of the IEEE, Vol. 77, No. 8, pp. 1208-1223 (1989)).
Since a pickup coil formed of a super-conducting wire is limited in a reduction ratio of the environmental magnetic field noise, there has been proposed a method of forming a pickup coil of a thin film for detecting a field gradient in one direction perpendicular to a detected magnetic field. In this event, a gradiometer detects (xcex94Bz/xcex94x) or (xcex94Bz/xcex94y), where Bz is a detected magnetic field (Prior Art 2: M. B. Ketchen, J. Appl. Phys., Vol. 58, No. 11, pp. 4322-4325 (1985)).
Also, for preventing the SQUID itself from picking up the environmental magnetic field noise, a known gradiometer comprises a differential SQUID configuration by arranging holes of the SQUID in series or in parallel (Prior Art 2 and Prior Art 3: R. S. Ahmad et al. Jpn. J. Appl. Phys., Vol. 36, Part 1, No. 11, pp. 6737-6741 (1997)).
Another known gradiometer has one of pickup coils constituting the gradiometer connected to the foregoing differential SQUID (Prior Art 4: K. Tsukada et al, Rev. Sci. Instrum., Vol. 66, No. 10, pp. 5085-5091 (1995)).
A further known gradiometer has one of pickup coils constituting the gradiometer and the differential SQUID fabricated on the same substrate (Prior Art 5: M. Koyanagi et al, IEEE Transactions on Magnetics, Vol. 25, No. 2, pp. 1166-1169 (1989)).
The foregoing exemplary gradiometers are configured to detect a field gradient (xcex94Bz/xcex94x) or (xcex94Bz/xcex94y) in one direction of the field. For estimating a source (current source) in a biomagnetometer, it is necessary to measure both (xcex94Bz/xcex94x) and (xcex94Bz/xcex94y) at the same time. To meet this requirement, there has been reported an element which has two gradiometers,fabricated on a single substrate, which detect field gradients rectangular to each other (Prior Art 6: 4D Neuroimaging, Product Document).
A gradiometer integrating pickup coils according to the present invention is fabricated of thin films on a substrate. In the following description, a rectangular coordinate system (x, y, z) is used, where a plane parallel with the surface of the substrate is defined as an xy-plane; and a direction perpendicular to the substrate as a z-axis. The gradiometer integrating pickup coils according to one embodiment of the present invention is configured to detect a gradient (xcex94Bz/xcex94x) of a magnetic field component (Bz) in the normal direction (z-direction) with respect to the x-direction and/or a gradient (xcex94Bz/xcex94y) with respect to the y-direction. First, terms used in the following description will be explained below.
xe2x80x9cA pickup coilxe2x80x9d refers to a planar pickup coil fabricated of a thin film on a substrate.
xe2x80x9cA center of a pickup coilxe2x80x9d refers to the center of the outer shape of the pickup coil. Here, the center of the pickup coil is defined as the center of gravity of an ideal plate having an outer shape equal to the outer shape of the pickup coil.
xe2x80x9cAn axial segment of a pickup coilxe2x80x9d refers to a segment connecting the centers of two pickup coils, which form part of a gradiometer, projected perpendicularly onto the surface of the substrate.
xe2x80x9cAn axial segment of a pickup coil in an x-directionxe2x80x9d refers to a segment connecting the centers of two pickup coils positioned in the x-direction in a gradiometer for the x-gradient, projected perpendicularly onto the surface of the substrate. The length of this axial segment is a baseline length of the gradiometer in the x-direction.
xe2x80x9cAn axial segment of a pickup coil in a y-directionxe2x80x9d refers to a segment connecting the centers of two pickup coils positioned in the y-direction in a gradiometer for y-gradient, projected perpendicularly onto the surface of the substrate. The length of this axial segment is a baseline length of the gradiometer in the y-direction.
xe2x80x9cThe center of a pickup coilxe2x80x9d refers to the midpoint on an axial segment of a pickup coil.
xe2x80x9cThe center of a pickup coil in the x-directionxe2x80x9d refers to the midpoint on the axial segment of a pickup coil in the x-direction.
xe2x80x9cThe center of a pickup coil in the y-directionxe2x80x9d refers to the midpoint on the axial segment of a pickup coil in the y-direction.
xe2x80x9cA differential SQUIDxe2x80x9d is a planar SQUID fabricated of a thin film on a substrate, the holes of which are in series or parallel differential configuration.
xe2x80x9cAn axial segment of a differential SQUIDxe2x80x9d refers to a segment connecting the center of a first superconducting loop (SQUID hole), forming part of a differential SQUID, with the center of a second superconducting loop (SQUID hole), likewise forming part of the differential SQUID, projected perpendicularly onto the surface of the substrate.
xe2x80x9cThe center of a differential SQUIDxe2x80x9d refers to the midpoint on the axial segment of the differential SQUID.
xe2x80x9cAn axial segment of a differential SQUID in the x-directionxe2x80x9d refers to the axial segment of the differential SQUID oriented in the x-direction, projected perpendicularly onto the surface of the substrate.
xe2x80x9cAn axial segment of a differential SQUID in the y-directionxe2x80x9d refers to the axial segment of the differential SQUID oriented in the y-direction, projected perpendicularly onto the surface of the substrate.
xe2x80x9cThe center of a differential SQUID in the x-directionxe2x80x9d refers to the midpoint on the axial segment of the differential SQUID in the x-direction.
xe2x80x9cThe center of a differential SQUID in the y-directionxe2x80x9d refers to the midpoint on the axial segment of the differential SQUID in the y-direction.
xe2x80x9cAn axial segment of a pickup coil is in alignment with an axial segment of a differential SQUIDxe2x80x9d means that the axial segment of a pickup coil overlaps the axial segment of the differential SQUID.
xe2x80x9cAn axial segment of a pickup coil in the x-direction is in alignment with an axial segment of a differential SQUID in the x-directionxe2x80x9d means that the axial segment of a pickup coil in the x-direction overlaps the axial segment of the differential SQUID in the x-direction.
xe2x80x9cAn axial segment of a pickup coil in the y-direction is in alignment with an axial segment of a differential SQUID in the y-directionxe2x80x9d means that the axial segment of a pickup coil in the y-direction overlaps the axial segment of the differential SQUID in the y-direction.
xe2x80x9cThe center of a gradiometer integrating pickup coilsxe2x80x9d (1) refers to a point at which the center of the pickup coil in the x-direction matches the center of the differential SQUID in the x-direction; (2) refers to a point at which the center of the pickup coil in the y-direction matches the center of the differential SQUID in the y-direction; and (3) refers to a point at which the center of the pickup coil in the x-direction, the center of the differential SQUID in the x-direction, the center of the pickup coil in the y-direction, and the center of the differential SQUID in the y-direction match one another.
The aforementioned Prior Art 1 has a problem of complicated installation, and inevitable errors included in a measured magnetic field due to environmental magnetic field noise introduced from wires used for the installation. In addition, a pickup coil formed of a wire has a problem of a limited mechanical accuracy and a limited reduction ratio of the environmental magnetic field noise.
Prior Arts 2, 3, 4 also have a problem left unsolved that environmental magnetic field noise is introduced from wires for installation, since the magnetic coils are not integrated with the SQUID, to inevitably cause errors in a measured magnetic field.
FIG. 1 is a diagram for generally explaining the configuration of a single-direction (x-direction) gradiometer according to the prior art which has two pickup coils and a differential SQUID fabricated on a single substrate. In Prior Art 5, an axial segment 22X of a pickup coil is not in alignment with an axial segment 21 of the differential SQUID (1X), and the centers of the pickup coils do not match the center of the differential SQUID, as shown in FIG. 1, so that Prior Art 5 experiences errors inevitably introduced into in a measured magnetic field. In addition, Prior Art 5 does not take into account a simultaneous measurement of x-gradient and y-gradient.
FIG. 2 is a diagram for generally explaining the configuration of two rectangular pickup coils in x-and y-direction of a gradiometer according to the prior art. Prior Art 6 forms two rectangular pickup coils 11Xp, 11Xn in the x-direction, and pickup coil 11Yp, 11Yn in the y-direction of the gradiometer, with the centers of the pickup coils in the x-direction matching the centers of the pickup coils in the y-direction. However, as shown in FIG. 2, Prior Art 6 fails to explicitly indicate the position at which differential SQUIDs in the x-direction and y-direction are formed corresponding to the pickup coils in the x-direction and y-direction. Also, Prior Art 6 has a problem that when a substrate on which the pickup coils are fabricated is separated from a substrate on which the SQUIDs are fabricated, so that environmental field magnetic noise is introduced from wires for installation, causing inevitable errors in a measured magnetic field. It should be noted that in FIG. 2, the pickup coils in the x-direction and y-direction are drawn in different line widths, and the centers of the pickup coils in the x-direction and y-direction are offset from each other for the ease of understanding.
The following three conditions must be satisfied for accurately detecting a magnetic field at a high sensitivity using pickup coils and differential SQUIDs fabricated of respective thin films on a substrate. However, any configuration satisfying the three conditions has not been known before.
(1) An axial segment of a pickup coil forming part of a gradiometer is in alignment with an axial segment of a differential SQUID.
(2) The center of a pickup coil forming part of an x-direction gradiometer, the center of a differential SQUID in the x-direction, the center of a pickup coil forming part of a y-direction gradiometer, and the center of a differential SQUID in the y-direction match one another.
(3) The pickup coils and differential SQUIDs are fabricated together on the same substrate to eliminate superconducting connections.
It is an object of the present invention to provide a gradiometer integrating pickup coils formed of thin films, which is capable of satisfying the foregoing three conditions, reducing the introduced environmental magnetic field noise, and detecting a magnetic field generated from a subject under testing at a high sensitivity. More particularly, it is an object of the present invention to provide a biomagnetometer which uses a multichannel gradiometer integrating pickup coils as a detector for purposes of measuring a biomagnetic field generated from neural activities of a human brain and myocardium activities of a human heart, and so on, and a magnetic field measurement system which uses the gradiometer integrating pickup coils as a detector for purposes of conducting a non-destructive evaluation.
The gradiometer integrating pickup coils according to the present invention may be implemented in two configurations: a flux transformer configuration, and a parallel pickup coil configuration. The flux transformer configuration transmits a magnetic field detected by a pickup coil to a SQUID through an input coil, where the pickup coil is not electrically connected to the SQUID. The parallel pickup coil configuration has parallelly connected superconducting loops which form a SQUID, where pickup loops are not strictly distinguished from the superconducting loops of the SQUID.
In a gradiometer integrating pickup coils in flux transformer configuration, an axial segment of a pickup coil is placed in alignment with an axial segment of a differential SQUID, with the center of the pickup coil (midpoint on the axial segment of the pickup coil) matching the center of the differential SQUID (midpoint on the axial segment of the SQUID). In this configuration, the pickup coil and differential SQUID are fabricated on the same substrate for eliminating superconducting connections.
For simultaneously measuring an x-gradient and a y-gradient, an x-direction gradiometer and a y-direction gradiometer are fabricated on the same substrate, with an axial segment of a pickup coil in the x-direction placed in alignment with an axial segment of a differential SQUID in the x-direction, an axial segment of a pickup coil in the y-direction placed in alignment with an axial segment of a differential SQUID in the y-direction, and the axial segment of the pickup coil in the x-direction and the axial segment of the pickup coil in the y-direction crossing at right angles. In addition, the center of the pickup coil in the x-direction, the center of the differential SQUID in the x-direction, the center of the pickup coil in the y-direction, and the center of the differential SQUID in the y-direction match one another. For eliminating superconducting connections, the pickup coil in the x-direction, pickup coil in the y-direction, differential SQUID in the x-direction and differential SQUID in the y-direction are all fabricated of thin films on the same substrate.
In a gradiometer integrating pickup coils in parallel pickup coil configuration, parallelly connected pickup coils form superconducting loops of a differential SQUID. Also, parallelly coupled pickup coils constituting a differential SQUID in the x-direction and parallelly coupled pickup coils constituting a differential SQUID in the y-direction are fabricated on a single substrate, such that an axial segment of the pickup coils in the x-direction and an axial segment of the pickup coils in the y-direction cross at right angles. Further, the center of the pickup coils in the x-direction matches the center of the pickup coils in the y-direction.
The gradiometer integrating pickup coils of the present invention can accurately measure a gradient (xcex94Bz/xcex94x) with respect to the x-direction or a gradient (xcex94Bz/xcex94y) with respect to the y-direction of a field component (Bz) in the normal direction in a simple design, irrespective of the configuration, with introduction of less environment magnetic field noise and less errors. In addition, the gradiometer integrating pickup coils of the present invention can simultaneously detect the gradient (xcex94Bz/xcex94x) with respect to the x-direction and the gradient (xcex94Bz/xcex94y) with respect to the y-direction.
The gradiometer of the present invention is particularly suitable for accurately detecting a feeble biomagnetic field.
The following description will be made on a first feature of the gradiometer integrating pickup coils in flux transformer configuration. A first and a second pickup coil substantially in the shape of square, and a differential SQUID are fabricated of thin films on the same substrate. The differential SQUID is magnetically coupled to the first and second pickup coils, and is formed of a first and a second superconducting loop connected in series or in parallel.
The first and second pickup coils and the first and second superconducting loops are fabricated of thin films on the same substrate so as to satisfy the following conditions (1), (2):
(1) A first segment resulting from projecting a segment connecting the center of the first pickup coil with the center of the second pickup coil perpendicularly onto the surface of the substrate overlaps a second segment resulting from projecting a segment connecting the center of the first superconducting loop with the center of the second superconducting loop perpendicularly onto the surface of the substrate.
(2) The midpoint of the first segment matches the midpoint of the second segment.
The first and second pickup coils constitute closed loops together with a first and a second input coil, respectively. The first pickup coil has the first input coil in a first loop for inputting magnetic flux generated by a first current induced in the first pickup coil by a magnetic field in a z-direction perpendicular to the surface of the substrate into the first superconducting loop. On the other hand, the second pickup coil has the second input coil in a second loop for inputting magnetic flux generated by a second current induced in the second pickup coil by the magnetic field in the z-direction into the second superconducting loop.
The gradiometer integrating pickup coils in the first feature is capable of detecting a field gradient in the z-direction with respect to the x-direction or y-direction, which is in parallel with the surface of the substrate and perpendicular to the z-direction. Since the pickup coils as well as the SQUID are fabricated of thin films, the gradiometer exhibits a high mechanical accuracy, and does not generate any error since the center of the pickup coils matches the center of superconducting loops of the SQUID.
The following description will be made on a second feature of the gradiometer integrating pickup coils in flux transformer configuration. A first, a second, a third, and a fourth pickup coil substantially in the shape of square, and a first and a second differential SQUID are fabricated of thin films on the same substrate. The first differential SQUID is formed of a first and a second superconducting loop connected in series or in parallel, and is magnetically coupled to the first and second pickup coils. The second differential SQUID is formed of a third and a fourth superconducting loop connected in series or in parallel, and is magnetically coupled to the third and fourth pickup coils.
The first, second, third and fourth pickup coils and the first, second, third and fourth superconducting loops are fabricated of thin films on the same substrate so as to satisfy the following conditions (1), (2), (3):
(1) A first segment resulting from projecting a segment connecting the center of the first pickup coil with the center of the second pickup coil perpendicularly onto the surface of the substrate overlaps a second segment resulting from projecting a segment connecting the center of the first superconducting loop with the center of the second superconducting loop perpendicularly onto the surface of the substrate.
(2) A third segment resulting from projecting a segment connecting the center of the third pickup coil with the center of the fourth pickup coil perpendicularly onto the surface of the substrate overlaps a fourth segment resulting from projecting a segment connecting the center of the third superconducting loop with the center of the fourth superconducting loop perpendicularly onto the surface of the substrate.
(3) The first segment and the third segment cross at right angles, and the midpoint of the first segment, the midpoint of the second segment, the midpoint of the third segment, and the midpoint of the fourth segment match one another.
The first, second, third and fourth pickup coils each form a closed loop together with a first, a second, a third and a fourth input coil associated therewith. The first pickup coil has the first input coil in a first loop for inputting magnetic flux generated by a first current induced in the first pickup coil by a magnetic field in a z-direction perpendicular to the surface of the substrate into the first superconducting loop. On the other hand, the second pickup coil has the second input coil in a second loop for inputting magnetic flux generated by a second current induced in the second pickup coil by the magnetic field in the z-direction into the second superconducting loop. The third pickup coil has the third input coil in a third loop for inputting magnetic flux generated by a third current induced in the third pickup coil by the magnetic field in the z-direction in the third superconducting loop. The fourth pickup coil has the fourth input coil in a fourth loop for inputting magnetic flux generated by a fourth current induced in the fourth pickup coil by the magnetic field in the z-direction in the fourth superconducting loop.
The gradiometer integrating pickup coils in the second feature is capable of simultaneously detecting field gradients in the z-direction with respect to the x-direction and the y-direction parallel with the surface of the substrate and perpendicular to the z-direction, in addition to the effects produced by the first feature.
The following description will be made on a third feature of the gradiometer integrating pickup coils in flux transformer configuration. A pickup coil formed of a first and a second pickup loop substantially in the shape of square, and a differential SQUID are fabricated of thin films on the same substrate. The pickup coil, fabricated of a thin film on the substrate, forms a closed loop together with an input coil in an 8-figured shape as a whole such that currents flow in the first and second pickup loops in directions opposite to each other, with respect to the application of a uniform field. The differential SQUID is magnetically coupled to the first and second pickup loops through the input coil, and is formed of a first and a second superconducting-loop connected in series or in parallel.
The pickup coil and the differential SQUID are fabricated of thin films on the same substrate so as to satisfy the following conditions (1), (2):
(1) A first segment resulting from projecting a segment connecting the center of the first pickup loop with the center of the second pickup loop perpendicularly onto the surface of the substrate overlaps a second segment resulting from projecting a segment connecting the center of the first superconducting loop with the center of the second superconducting loop perpendicularly onto the surface of the substrate.
(2) The midpoint of the first segment matches the midpoint of the second segment.
The gradiometer integrating pickup coils in the third feature is capable of detecting a field gradient in the z-direction with respect to the x-direction or the y-direction which is in parallel with the surface of the substrate and perpendicular to the z-direction. Since the pickup coil as well as the SQUID are fabricated of thin films, the gradiometer exhibits a high mechanical accuracy, and does not generate any error since the center of the pickup coil matches the center of superconducting loops of the SQUID.
The following description will be made on a fourth feature of the gradiometer integrating pickup coils in flux transformer configuration. A first and a second pickup coil and a first and a second differential SQUID are fabricated of thin films on the same substrate. The first pickup coil is fabricated of a thin film on the substrate such that a first and a second pickup loop substantially in the shape of square form a closed loop together with input coils in an 8-figured shape as a whole such that currents flow in the first and second pickup loops in directions opposite to each other. The second pickup coil is fabricated of a thin film on the substrate such that a third and a fourth pickup loop substantially in the shape of square form a closed loop together with input coils in an 8-figured shape as a whole such that the current flow in the third and fourth pickup loops in the direction opposite to each other.
The first differential SQUID is magnetically coupled to the first and second pickup loops through the input coils, and is formed of a first and a second superconducting loop connected in series or in parallel. The second differential SQUID is magnetically coupled to the third and fourth pickup loops through the input coils, and is formed of a third and a fourth superconducting loop connected in series or in parallel.
The first, second, third and fourth pickup loops and the first, second, third and fourth superconducting loops are fabricated of thin films on the same substrate so as to satisfy the following conditions (1), (2), (3) and (4):
(1) A first segment resulting from projecting a segment connecting the center of the first pickup loop with the center of the second pickup loop perpendicularly onto the surface of the substrate overlaps a second segment resulting from projecting a segment connecting the center of the first superconducting loop with the center of the second superconducting loop perpendicularly onto the surface of the substrate.
(2) A third segment resulting from projecting a segment connecting the center of the third pickup loop with the center of the fourth pickup loop perpendicularly onto the surface of the substrate overlaps a fourth segment resulting from projecting a segment connecting the center of the third superconducting loop with the center of the fourth superconducting loop perpendicularly onto the surface of the substrate.
(3) The first segment and the third segment cross at right angles.
(4) The midpoint the first segment, the midpoint of the second segment, the midpoint of the third segment, and the midpoint of the fourth segment match one another.
The gradiometer integrating pickup coils in the fourth feature is capable of simultaneously detecting field gradients in the z-direction with respect to the x-direction and the y-direction parallel with the surface of the substrate and perpendicular to the z-direction, in addition to the effects produced by the third feature.
The following description will be made on a fifth feature of the gradiometer integrating pickup coils in parallel pickup coil configuration. A first and a second pickup coil formed of superconducting loops and connected in series or in parallel, and a differential SQUID formed of a plurality of superconducting loops respectively connected in parallel with the first and second pickup coils are fabricated of thin films on a substrate. The first pickup coil and a plurality of superconducting loops connected in parallel therewith, and a second pickup coil and a plurality of superconducting loops connected in parallel therewith are fabricated of thin films on the same substrate symmetrically about the x-axis and the y-axis which are parallel with the surface of the substrate and perpendicular to the z-direction.
The gradiometer integrating pickup coils in the fifth feature is capable of detecting a field gradient in the z-direction with respect to the x-direction or the y-direction which is in parallel with the surface of the substrate and perpendicular to the z-direction. Since the pickup coils as well as the SQUID are fabricated of thin films, the gradiometer exhibits a high mechanical accuracy.
The following description will be made of a sixth feature of the gradiometer integrating pickup coils in parallel pickup coil configuration. A first and a second differential SQUID are fabricated of a thin film on the same substrate. The first differential SQUID, connected to a first and a second pickup coil, each formed of a superconducting loop, in series or in parallel, is formed of a plurality of superconducting loops which are connected respectively in parallel with the first and the second pickup coils. The second differential SQUID, connected to a third and a fourth pickup coil, each formed of a superconducting loop, in series or in parallel, is formed of a plurality of superconducting loops which are connected respectively in parallel with the third and fourth pickup coils.
The first and second differential SQUIDs are fabricated of a thin film on the same substrate so as to satisfy the following conditions (1), (2):
(1) A first segment resulting from projecting a segment connecting the center of the first pickup coil with the center of the second pickup coil perpendicularly onto the surface of the substrate, and a second segment resulting from projecting a segment connecting the center of the third pickup coil with the center of the fourth pickup coil perpendicularly onto the surface of the substrate cross at right angles.
(2) The midpoint of the first segment matches the midpoint of the second segment.
The gradiometer integrating pickup coils in the sixth feature is capable of simultaneously detecting field gradients in the z-direction with respect to the x-direction and the y-direction parallel with the surface of the substrate and perpendicular to the z-direction.
The gradiometer integrating pickup coils of the present invention can be fabricated on a single substrate in a simple structure, and readily installed within a cryostat.